1. Field of the Invention
This invention relates to a conveyor means for conveying articles at a production line, etc., particularly to a conveyor means using linear motors.
2. Description of the Prior Art
From the viewpoint of increasing the production efficiency and improving the working environment, it has been desired recently at the production lines of manufacturing plants to convey articles speedily and silently. A conveyor means developed to meet such requirements using linear motors, each comprising linear coils and a reaction member, is known. Such conveyor means has the following construction.
As shown in FIG. 16, linear motor coils 92 which, together with a reaction member 93, compose a linear motor 91 are made stators or fixed elements, which are arranged along a roller conveyor 95 comprising a plurality of rollers 94, and the reaction member 93 comprises a rotor or a moving element which is fitted to a pallet 96 (an article to be conveyed) through the medium of a fitting member 97. By thrust F which is generated at the rotor (reaction member 93) due to electromagnetic action between the linear motor coils 92 and the reaction member 93, the pallet 96 and an article placed thereon are conveyed through the medium of the rotor (reaction member 93). In the conveyor means using linear motors of this kind, a controller is generally provided for the linear motor 91 of each working station and when the pallet 96 is conveyed between adjoining stations, a conveying speed for the pallet 96 detected by an encoder provided at each station is inputted to the controller for controlling the working of the linear motor 91.
Also, the Japanese Patent Application Laying Open Gazette No. 55-86307 discloses conveyor means using a DC servomotor of direct driving system using gears, etc. at each station for stopping an article being conveyed exactly at the specified position of each station. According to this conveyor means, when an article being conveyed approaches each station, driving by a linear motor is stopped and switched over to the driving by the servomotor so as to convey the article being conveyed to the stop position at each station.
In comparing the thrust-speed characteristic between the linear motor and the servomotor (rotary motor), as shown in FIG. 17, while the linear motor generates large thrust in a low speed region at starting, the servomotor generates large thrust in a high speed region after the lapse of the specified time from starting. From this fact, it is desirable for the driving means of a conveyor means to use a linear motor in a low speed region (an acceleration region where acceleration at the initial stage of conveyance is required and a deceleration region where deceleration at the last stage of conveyance is required) but to use a servomotor in high speed region where the speicfied speed (high speed) is maintained after acceleration, as shown in FIG. 18.
However, since the above linear motor is controlled in working on the basis of a conveying speed for the pallet grasped indirectly by pressing an encoder against the pallet through the medium of a roller or the like, there is caused slip between the pallet and the roller and therefore the control is substantially an open-loop control system and fluctuations of conveying speed in conveying a pallet can be caused as shown by a virtual line (two-point chain line) in FIG. 18, namely, there is caused a deviation at each speed change point (from the low speed region or the acceleration region by a linear motor to the high speed region or the fixed speed region by a servomotor and from the high speed region by a servomotor to the low speed region or the deceleration region by a linear motor). Therefore, the deceleration starting time varies to recover the conveying distance affected by the deviation at the speed change point. As a result, the conveying length of time stop position becomes irregular and control precision on conveyance as a whole becomes worse.
When a linear motor is used in the low speed region (acceleration region and deceleration region) where the speed control is difficult, the linear motor is controlled in working by a controller on the basis of the pallet conveying speed from the encoder and therefore a controller which is equipped with a complicated control apparatus must be used and a load on the control system (controller) of the linear motor becomes large, with the result that reliability of the control system of the linear motor lowers and the control system is easy to break down.